The determination of the steering center of a moving vehicle is important for several advanced vehicle functions, such as those run by the vehicle dynamic controller, i.e. vehicle traction control. It is important to learn this value as early in the driving cycle as possible so that these vehicle functions may be activated in a timely manner.
One known way of determining the steering center of a moving vehicle is the use of an absolute steering wheel angle sensor. An absolute steering wheel angle sensor provides the actual position of the steering wheel at any point in the vehicle's driving cycle. Unfortunately, these sensors are complex and costly.
Another method utilizes relative steering wheel angle sensors, which are less complex and costly as compared with the absolute steering wheel angle sensors. This type of sensor does not provide the absolute position of the steering wheel. A relative steering wheel angle sensor (RSAS) generates a relative steering wheel angle, that is, relative to the initial position which the steering mechanism occupied when the ignition was turned on. Since it cannot be guaranteed that a vehicle is always started with the steering mechanism in the straight-ahead position, it becomes necessary to estimate the location of the center position. When the center position is located, an RSAS offset value may be computed which will be used to adjust the measured value given by the relative sensor (RSAS).
Accordingly, after the vehicle is started, the electrical control unit (ECU) calculates the center position, and hence the initial RSAS offset. This calculation requires the vehicle to be moving at a certain speed range and in a straight-ahead manner. Calculating the offset depends on the vehicle speed, the road surface condition, and the driving maneuver. Unfortunately, during this “calculation” time period the vehicle dynamic controller is temporarily disabled.
Another significant drawback occurs between ignition on/off cycles. Storing the calculated RSAS offset value to memory during ignition turn-off is not satisfactory, because the ECU always treats the RSAS reading of the steering mechanism as at zero degrees when the ignition turns on. Since the steering wheel is not necessarily in the original position of the prior cycle when the ignition is turned off, this value cannot be used. Thus, the ECU usually stores the last steering wheel angle (SWA) actual position to memory when the ignition turns off. Unfortunately, if the ECU utilizes this saved SWA when the ignition is turned on, the calculated “center” position and the estimated RSAS offset could be erroneous since the steering wheel may have been moved while the ignition was turned off.
Therefore, there exists the need to provide a steering wheel angle sensor that quickly provides a highly accurate value of the steering wheel angle so that advanced vehicle functions may be readily activated, while also providing cost savings to lower the overall cost of the vehicle.